1. Field of the Invention
The present imvention relates to a device for turning a sheet of paper and, more specifically, to such a device usable for copying on both sides of a sheet of paper in a copying machine.
2. Disclosure of the Background Art
An image formation apparatus, such as a copying machine or a laser printer, capable of forming an image on both sides of a sheet of paper is commercially available. In such an image formation apparatus, the sheet must be turned. For that purpose, a switchback type sheet turning device as shown in FIG. 6, for example, is commonly employed. The sheet turning apparatus is provided with a sheet transporting path 52, which makes a loop-shaped path with a printing unit 51 therein, to transport the sheet in the counterclockwise direction in the figure. The sheet is turned in the course of the circulation in the sheet transporting path 52 by rollers 53 which rotates in both the forward and reverse directions. Specifically, after printed with an image on one side in the printing unit 51, the sheet is transported to the rolllers 53 and moved by the rollers 53 until the tail portion of the sheet is nipped by the rollers 53. Then, the sheet is stopped once and transported with its original tail portion first by reverse rotation of the rollers 53. Since the sheet is turned by the transportation through which the original tail portion of the sheet goes first, the other side of the sheet is then formed with an original image when the sheet is transported through the printing unit 51 again. After that, the sheet with two images on both sides is discharged onto a sheet receiver 54 by the forward rotation of the rollers 53.
When a two-sided copy is carried out with the above device turning the sheet, the directions of the images on both sides of the sheet may be opposite to each other. For example, in the case where a direction of the sheet transportation is the same as that of the images, an image shown in a solid line in FIG. 7 is made on the first face of the sheet which is transported in the direction of an arrow A shown in FIG. 7. However, after the rollers 53 turns the sheet so that the tail of the sheet goes first, the sheet is transported in direction of an arrow B. Therefore, the direction of the image formed on a second face of the sheet is opposite to that of the image on the first face as can be seen from an illustration of a broken line in FIG. 7.
In order to solve the above disadvantage, image information stored in memory can be read from the end of the information in, for example, a laser printer. In this manner, however, the control must be complicated.
Transporting a sheet in a direction orthogonal to the transporting direction may come to turn the sheet as proposed in Japanese Examined Patent Publication No. 22787/1980. According to the description of the publication, first a sheet having an image on one side is turned so that the first character train in the image goes first, that is, the sheet is turned in a direction opposite to the predetermined direction when the image is automatically copied on both sides of the sheet through a copying machine. Second after the sheet goes through a fuser, the sheet is received by the sheet turning device. In the device, the sheet is transported in a direction orthogonal to the predetermined direction so that one side of the sheet goes first, and then the sheet is turned. Third, the sheet is transported in the predetermined direction so that the original head of the sheet goes first, and then the sheet is turned. Fourth, the sheet is transported in order to copy an image on the second face of the sheet. Fifth, the sheet having two images on both sides is received from the fuser by the sheet turning device and turned. After that, the sheet is transported to a sorter.